Superconductors are materials which lack measurable electrical resistivity below a transition temperature (i.e., critical temperature, T.sub.c). Materials having a T.sub.c of 20 K and above are generally referred to as high-temperature superconductors (HTS). Superconducting materials also exhibit a critical current, I.sub.c, which is the current at a specified temperature (and in the absence of external magnetic fields) above which the material is normal and below which the material is superconducting.
In order to obtain better mechanical properties, it is common to use composites of HTS materials and metals rather than using superconducting materials alone. These composites may be prepared in elongated forms such as wires and tapes by a well-known process which includes the three stages of: (i) forming a powder of superconductor precursor material (precursor powder formation stage); (ii) filling a metal container, such as a tube, billet or grooved sheet, with the precursor powder, and deformation processing one or more filled containers to provide a composite of reduced cross-section including one or more cores of superconductor precursor material in a surrounding noble metal matrix (composite precursor fabrication stage); and (iii) subjecting the composite to successive physical deformation and annealing cycles and further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing). The alignment of precursor grains in the core ("textured" grains) caused by the deformation process facilitates the growth of well-aligned and sintered grains of the desired superconducting material during later thermal processing stages.
The general process, commonly known as "powder-in-tube" or "PIT," is practiced in several variants depending on the starting powders, which may be, for example, metal alloys having the same metal content as the desired superconducting core material in the "metallic precursor" or "MPIT" process, or mixtures of powders of the oxide components of the desired superconducting oxide core material or of a powder having the nominal composition of the desired superconducting oxide core material in the "oxide powder" or "OPIT" process. General information about the PIT method described above and processing of the oxide superconductors is provided by Sandhage et al., in JOM, Vol. 43, No. 3 (1991) pages 21-25, and the references cited therein. Sandhage et al., in JOM, Vol. 43, No. 3 (1991) pages 21-25 is incorporated herein by reference.
As an example of the PIT method, a (Bi,Pb)SCCO precursor powder may be packed into a silver sheath to form a billet. The billet is extruded to a diameter of about 1/3 of the original diameter and then narrowed with multiple die passes. A mono-filamentary tape is fabricated by further extrusion and/or drawing of the billet to a wire, and then rolling the wire, for example, to a 0.006".times.0.100" tape. Alternatively, a multi-filamentary tape may be fabricated by multiple die passes through hexagonally shaped dies of varying sizes to form a silver sheathed (Bi,Pb)SCCO hexagonal wire. Several of the hexagonal wires may bundled together and drawn through a round die to form a multi-filamentary round wire. The round wire may then be rolled, for example, to form a multi-filamentary silver and (Bi,Pb)SCCO composite precursor tape of a width of about 0.080" to 0.200" and a thickness of about 0.004" to 0.010". The composite may be textured using by one or more texturing deformation steps.
It is often desirable to have long lengths (e.g., 1000 m) of such composites in the form of a wire or the like. For example, in the context of power transmission lines, it is desirable to wrap long lengths of HTS wire. Consequently, it is frequently necessary to splice composite wires together in order to form a joined wire of sufficient length.
U.S. Pat. Nos. 5,116,810 and 5,321,003, both to Joshi et al., are directed to processes and products for making electrical connections between superconducting elements, by joining metallic precursor elements prior to conversion to the oxide superconductor. Since the joints are formed prior to the oxidation process, joints formed in accordance with these patents are substantially non-resistive in nature. However, it is frequently inconvenient to perform such joining operations on HTS precursors, rather than on components in the superconducting state.
The inventors faced the problem of avoiding the disadvantages rising from joining superconducting components, including local strain concentration of the wire as a result of the thickness gradient of the joint and of the winding process. Prior art techniques have been deficient in that the joined wires used often suffer from significant critical current degradation. Wires formed by these techniques also experience further critical current degradation as a result of local strain concentrations during the winding process.
It would therefore be desirable to provide methods for joining HTS components such that critical current degradation is negligible or minimized, thereby overcoming the shortcomings associated with the prior art.